Field of Expertise: Advanced Material Science
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Field of Expertise: Advanced Material Science | |
Temperature evolution in soft matter materials during focused ion beam prototyping: from fundamentals towards optimization During the last decade focused ion beam (FIB) processing became of increased importance due to its unique possibilities of site specific specimen manipulation. So far, FIB processing has been well established for transmission electron microscopy related ultrathin lamella preparation, but also gained importance as a method for 3D metrology and 3D surface structuring from the micro- to the nanoscale. Due to its flexibility and straightforward implementation character, it represents a rapid prototyping tool for science and technology. Aside from these undoubted advantages, FIB processing entails unwanted side effects, such a spatially confined ion implantation, surface or bulk amorphization and partial high thermal stress. While the former two are intrinsic properties and therefore invariable, local heating effects have been shown to depend strongly on the patterning strategy. The reduction of this technically induced heating is of particular relevance for low melting materials such as polymers and biological material but requires an improved understanding of the thermal effects during scanning. As a starting point we discuss briefly a recently introduced alternative patterning strategy, which strongly reduced local temperatures by a decoupling from technically induced effects. As a second point different polymers are subjected to this alternative process and compared to classical strategies. Characterization includes morphology via scanning electron microscopy (SEM), atomic force microscopy (AFM) and Kelvin force microscopy (KFM) as well as chemistry via Raman spectroscopy (RS). In this contribution we focus on simulation based comparisons of thermal effects between Si and polymers and its evolution during the patterning process. It will be shown that the low thermal conductivity of polymers is one of the key parameters responsible for massive local heating during classical raster scan procedures. The simulations are then expanded to special scan strategies to confine the origin of this technically induced additional heating by a decoupling from the ion beam related interaction volume. Finally, a simple alternative patterning strategy is presented which reduces these thermal effects massively, approaching the intrinsic (and unavoidable) limitation given by a single ion beam pulse. The study demonstrates the capabilities of FIB processing for low melting materials by using adapted scan strategies which can be easily implemented in most FIB systems. By that, new possibility for FIB based rapid prototyping on low melting materials might open up which have been considered as very complicated or even impossible before. The simulations are then compared to experiments with different polymers such as high-density polyethylene (HDPE) and polymethylmethacrylate (PMMA). Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy are used for the morphological and chemical characterization in dependency on FIB process parameters such as point pitches (PoP), dwell times (DT), total exposure times (TET), patterning sizes and scan strategies at room temperature and cryogenic conditions (-150 °C). The results are in well agreement with the simulations and prove the technically induced additional heating during standard patterning as significant and highly degrading contribution. Furthermore, the strong improvements by the alternative patterning strategy are demonstrated which allow for increasing morphological stability (flat instead of often rugged bottom areas) and decreasing chemical degradation (maintained functionality). |